The present invention is directed to a process for forming highly textured bulk objects (including single-crystals), and extended objects (such as wires and tapes) by applying a strong magnetic field (H.gtoreq.10.sup.4 Oe) and high (near melting point) temperatures to materials having anisotropy in the paramagnetic or diamagnetic susceptibility (.chi.).
Standard conductors obey Ohm's law. As charges move through a conductor they produce an electric field that creates an electric current. The current density (J) of a conductor having a cross-sectional area A is defined as follows: EQU J=I/A
where I is the current running through the conductor. If a constant potential difference is maintained across a conductor the current (I) remains constant. If the current density in a conductor is proportional to the electric field within the conductor, the conductor is said to obey Ohm's law: EQU J=.sigma.E
where .sigma. is the constant of proportionality and E is the electric field strength.
The resistance of a conductor may be calculated according to the following expression: EQU R=V/I
where V is the potential difference, and the resistivity of a material is found according to the following expression: EQU .rho.=1/.sigma..
In the early part of this century it was discovered that resistance dropped to zero in some materials, such as mercury (Hg), at low temperatures. Materials having an electrical resistance of zero (R=0) are called superconductors. Such materials do not obey Ohm's law.
Theories such as the BCS Theory of Superconductivity and the Cooper Electron Pairing Theory attempt to explain the lack of resistance in superconducting materials at T.sub.c (temperature at which resistance equals zero). While the BCS theory has not been entirely successful with new high T.sub.c ceramics the Cooper Electron Pairing Theory is commonly considered valid.
Recent developments in superconductor technology such as Paul Chu's synthesis of a material superconducting at 98K (YBa.sub.2 Cu.sub.3 O.sub.x, where x.apprxeq.7) and then the recent discovery of a superconducting ceramic material (TlBa.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+3) with a T.sub.c of .apprxeq.125K have lead to promising directions.
Now that economical T.sub.c 's have been achieved new problems must be confronted and solved. One of these problems is critical current density (J.sub.c).
For technological applications the critical current density (maximum current above which a material ceases to be superconducting) should have a magnitude of about 10.sup.5 to 10.sup.6 A/cm.sup.2 at 77K. In single-crystals RE--Ba.sub.2 Cu.sub.3 O.sub.x (where RE is a rare earth) a critical current (J.sub.c) of .apprxeq.3.times.10.sup.6 A/cm.sup.2 has been obtained. See, S. Jin, et al. 51 Applied Physics Letters 203 et seq. (1987).
However, J.sub.c is strongly anisotropic and is sufficient only for certain directions (for current flows in the Cu-O basal plane). In polycrystalline un-textured bulk and elongated samples, where grains are randomly oriented, superconducting current flows along "good" directions in some grains and along "bad" directions in other grains which results in an unacceptably low J.sub.c (10.sup.2 to 10.sup.3 A/cm.sup.2). Grain boundaries also have an adverse effect on J.sub.c.
One logical approach to enhancing J is to prepare grain-oriented polycrystalline ceramics, or to turn or regrow grains in such a way that current flows along "good" directions only. Though the grain boundary problem or mismatch in a(b) axes registry may persist, it is possible to attain reasonably high J.sub.c 's in textured compacts.
Textured material is a material in which the vast majority of the grains within the material have the same crystallographic orientation with respect to some reference direction. The highest probability texture direction is called the "preferred orientation."
A texture is specified with respect to the external directions of the material under consideration, for example, to the plane and edges of a tetragonally shaped bulk sample, or to the axis of a wire.
Textured materials very often have superior mechanical, electromagnetic, wave alternating and transducing properties, etc., and the demand for such material is increasing. For example, in the area of high temperature superconducting ceramics (HTSC) large single-crystal and high grain-oriented materials are desirable because they exhibit high critical current densities as well as high critical magnetic fields favored in single crystals and textured samples.
One method of producing texture is to influence the grain growth process. As grain growth occurs in a material, some grains grow at the expense of their neighbors. If a means can be found to enhance the growth of grains selected on the basis of their crystallographic orientation, highly textured material in which the vast majority of the grains are crystallographically oriented may be obtained.
A common method of producing such a selection mechanism is to utilize a temperature gradient during grain growth for materials with a large anisotropy in crystal growth directions. Another approach is to utilize mechanical pressure. For example, rolling thin sheets of certain metals causes preferential grain alignment.
In the present invention, the method of selecting a favored crystallographic orientation during grain growth includes providing a difference in the magnetic component of energy between grains favorably and unfavorably oriented with respect to the direction of an applied magnetic field. This difference in energy is due to two factors: (1) anisotropy in the paramagnetic/diamagnetic susceptibility (the difference in the grain magnetic susceptibilities in the directions parallel and perpendicular to the magnetic field); and (2) magnitude of the magnetic field itself (the energy term when the atomic magnetic moments are not saturated is proportional to the square of the magnetic field).
This implies that in order to maximize the magnetic energy term it is necessary to use the maximum achievable magnetic field. Magnetic fields of 10.sup.4 to 10.sup.5 Oe are currently producible with commercially available equipment.
When a material with an anisotropic magnetic susceptibility is placed in a magnetic field the energy of grains favorably oriented with respect to the field direction is lower than that of other orientations.
When a material is heated to a near melting point temperature and grain growth occurs, the larger size grains, due to a surface energy term, expand at the expense of the smaller grains. This process does not generally result in a preferred orientation. However, if a material is placed in a sufficiently strong magnetic field, so that the magnetic term dominates the surface energy term, the grains with favorable crystallographic orientation will grow at the expense of adjacent unfavorably oriented grains without regard to their size. This process results in textured samples.